Lithium Difluorophosphate‐Based Dual‐Salt Low Concentration Electrolytes for Lithium Metal Batteries

Abstract

The safety hazards and low Coulombic efficiency originating from the growth of lithium dendrites and decomposition of the electrolyte restrict the practical application of Li metal batteries (LMBs). Inspired by the low cost of low concentration electrolytes (LCEs) in industrial applications, dual‐salt LCEs employing 0.1 m Li difluorophosphate (LiDFP) and 0.4 m LiBOB/LiFSI/LiTFSI are proposed to construct a robust and conductive interphase on a Li metal anode. Compared with the conventional electrolyte using 1 m LiPF6, the ionic conductivity of LCEs is reduced but the conductivity decrement of the separator immersed in LCEs is moderate, especially for the LiDFP–LiFSI and LiDFP–LiTFSI electrolytes. The accurate Coulombic efficiency (CE) of the Li||Cu cells increases from 83.3% (electrolyte using 1 m LiPF6) to 97.6%, 94.5%, and 93.6% for LiDFP–LiBOB, LiDFP–LiFSI, and LiDFP–LiTFSI electrolytes, respectively. The capacity retention of Li||LiFePO4 cells using the LiDFP–LiBOB electrolyte reaches 95.4% along with a CE over 99.8% after 300 cycles at a current density of 2.0 mA cm−2 and the capacity reaches 103.7 mAh g−1 at a current density of up to 16.0 mA cm−2. This work provides a dual‐salt LCE for practical LMBs and presents a new perspective for the design of electrolytes for LMBs.

Confronting the Challenges in Lithium Anodes for Lithium Metal Batteries

“…[ 64 , 68 ] Therefore, various salts are probed to obtain the ideal SEI. Recently, dual‐salt or tri‐salt systems, such as, lithium difluorophosphate‐lithium bis(oxalato)borate (LiDFP‐LiBOB), [ 18 ] lithium bis(trifluoromethanesulfonyl)imide‐lithium bis(oxalato)borate (LiTFSI‐LiBOB), [ 69 ] lithium bis(trifluoromethanesulfonyl)imide‐lithium bis(oxalato)borate‐lithium hexafluorophosphate (LiTFSI‐LiBOB‐LiPF 6 ), [ 68 ] become a thriving trend and the synergistic effect among different salts has been verified beneficial to form SEI with favorable components and proper proportion of these components. In a dual salt (LiDFP‐LiBOB) system, the Li//LiFePO 4 battery operated stably for 300 cycles at the current density of 2 mA cm −2 .…”

“…Li 2 BO x , the decomposition products of LiBOB, contributed to suppressing the dissolution of the organic species in SEI and enhanced its flexibility. [ 18 ] Jin et al. [ 68 ] confirmed that lithium symmetric battery was more stable in a tri‐salt system (LiTFSI‐LiBOB‐LiPF 6 ) than in LiPF 6 or dual‐salt (LiTFSI‐LiBOB) systems ( Figure 2 a ), which was due to that tri‐salt system facilitated the formation of high‐quality SEI that contains sufficient both Li 2 CO 3 and ROCO 2 Li.…”

“…Among them, the dual‐salt and tri‐salt electrolyte systems play synergistic effect in building stable SEI. [ 18 ] The design of the organic/inorganic hybrid solid electrolyte and the electrolyte/electrode interface layer improve the contact between the solid electrolyte and the lithium anode. [ 19 , 20 ] A variety of methods guiding Li + (such as, lithiophilic sites, gradient electrical conductivity, reconstructed lattice plane) contribute to the uniform and compact deposition of lithium in the 3D host.…”

Advanced Electrolyte Design for High‐Energy‐Density Li‐Metal Batteries under Practical Conditions

“…It should be noted that the external pressure is the key factor to realize the excellent electrochemical performance of the anode-free Li-metal batteries.T he electrochemical performance of anode-free Li-metal batteries is the best ever reported. [86] When athin layer of graphite is coated on the Cu current collector to form aL i-ion/Li metal battery, [87] ap rolonged cycle life is achieved.…”

Electrolytes for Lithium‐ and Sodium‐Metal Batteries

“…As stated above, SEI film plays a crucial role in determining the property of LMBs; the stable and compact SEI film can effectively repress the side reaction and dendrite growth. The components and structure of SEI strongly depend on the dissociation degree of salts, different salts concentration electrolytes, [15,16] the stability of different solvents, [17] and the strength of the solvation of electrolytes. [18] Therefore, many additives are used to assist in the formation of stable SEI film on Li metal surface, such as fluoroethylene carbonate (FEC), LiNO 3 , ionic liquid, and Lithium chloride.…”